This invention relates to a polydisperse propylene polymer which is large in a ratio of a weight-average molecular weight to a number-average molecular weight (Mw/Mn). More particularly, the invention relates to a polydisperse propylene polymer having a low melting point which is suitable for the formation of the products excellent in stiffness and heat resistance, and also a process for producing the same by a multistage polymerization in the presence of a supported metallocene catalyst.
Propylene polymers have been extensively used in the field of various moldings, because of their good mechanical properties, good chemical resistance and very good balance between their properties and economy. These propylene polymers have been produced by homopolymerization of propylene or copolymerization of propylene and other olefins using a Ziegler-Natta catalyst system comprising an inorganic transition metal catalyst component having titanium trichloride, titanium tetrachloride or the mixture thereof supported on a support such as magnesium chloride, in combination with an organoaluminum compound.
In recent years, processes for the production of olefin polymers have been proposed wherein olefins such as propylene are polymerized using a catalyst comprising an organic complex of a transition metal, so-called metallocene in combination with an aluminoxane, the catalyst being different from a Zigler-Natta catalyst system. For example, JPA 61-130314, JPA 3-12406, JPA 3-12407, JPA 6-100579, U.S. Pat. No. 5,296,434 and Organometallics 13, 957 (1994), etc. disclose metallocene catalysts providing isotactic propylene polymers.
Olefin polymers produced using these metallocene catalysts are generally polymers having a relatively uniform molecular weight wherein a ratio of a weight-average molecular weight to a number-average molecular weight (Mw/Mn) is about 3 or lower. In the production of copolymers, metallocene catalysts can produce more homogeneous olefin copolymers than a supported Ziegler-Natta catalyst system, since comonomers are copolymerized homogeneously.
In general, there is a correlation between the physical properties of propylene polymers, in particular, the stiffness and heat resistance, and the melting point. The Higher melting point indicates the higher stiffness and heat resistance.
On a relationship between a polymerization temperature and a melting point of polypropylene where the catalyst providing an isotactic polypropylene is used, Journal of Molecular Catalysis A: Chemical 102, 59-65 (1995) describes that a high-melting polypropylene is produced, e.g., at 0xc2x0 C. but the melting point of polypropylene produced at an industrial polymerization temperature, e.g., at 70xc2x0 C. is extremely low. Organometallics 13, 957 (1994) describes that a high-melting polypropylene is produced even at an industrial polymerization temperature, 70xc2x0 C. by choice of the species of metallocene compounds in the polymerization in a laboratory scale wherein no catalyst is supported on a support, a so-called homogeneous polymerization.
For the industrial production of propylene polymers, it has been required to use a supported metallocene catalyst wherein a metallocene compound is supported on a support. WO 94/28034 discloses that when propylene is polymerized using a supported catalyst wherein the above metallocene compound providing a high-melting propylene polymer in a high-temperature polymerization is supported on a support, the melting point of the resultant polypropylene lowers as compared with polypropylene produced in a homogeneous polymerization.
Accordingly, it was difficult to industrially produce a high-melting isotactic polypropylene having high stiffness and heat resistance using the supported metallocene catalyst.
JPA 5-140227 discloses a process of producing polyolefins which are large in a ratio of a weight-average molecular weight to a number-average molecular weight using a plurality of metallocene compounds. Also, WO 94/16009 discloses a propylene polymer composition having good mechanical properties which comprises two propylene polymers with different melt flow rates (MFR).
An object of the invention is to provide a propylene polymer giving a molded product which has high stiffness and excellent heat resistance.
Another object of the invention is to provide a process of producing the propylene polymer by a multistage polymerization using a single transition metal compound.
The present inventors have found that a propylene polymer produced by polymerizing propylene by stepwise changing the polymerization conditions in each step in a multistage polymerization using a single transition metal compound exhibits a polydispersity and is excellent in moldability at low temperatures, and the molded articles made therefrom have high stiffness and excellent heat resistance.
The present invention is directed to a polydisperse propylene polymer characterized by being produced by polymerizing a propylene monomer or a mixed monomer of propylene and other olefin(s) by a multistage polymerization wherein the polymerization conditions are stepwise changed in the presence of a supported metallocene catalyst having a transition metal compound and an aluminoxane or a reaction product thereof supported on a finely particulate support and an organoaluminum compound, and by having a ratio of a weight-average molecular weight to a number-average molecular weight (Mw/Mn) of 4.0 or higher, a melting point (Tm) of 165xc2x0 C. or lower and a difference between the melting point (Tm) and a heat distortion temperature (HDT) of 30xc2x0 C. or less.
Another invention is directed to a process of producing the polydisperse propylene polymer, characterized by that a propylene monomer or a mixed monomer of propylene and other olefin(s) is polymerized by a multistage polymerization wherein the polymerization conditions are stepwise changed in the presence of a supported metallocene acatalyst having a transition metal compound and an aluminoxane or a reaction product thereof supported on a finely particulate support and an organoaluminum compound, the transition metal compound being selected from the group consisting of titanocene, zirconocene and hafnocene.